During an exposure process of a lithography machine, a wafer is divided into multiple fields of view to be exposed respectively. During an exposure process of a field of view on the wafer, an exposure image is projected onto the wafer by the lens to form an exposure region. In order to ensure the exposure accuracy, the wafer stage is required to be leveled and focused, and to adjust the horizontal position, so as to ensure the relative position and angle between the exposure region and the lens. Thus, six-degree-of-freedom displacement measurement of the exposure region is particularly important. When this field of view is exposed, the wafer stage moves, so that the next field of view is moved to the exposure region. Throughout the exposure process, the position of exposure region on the wafer is constantly changing.
In the existing laser interferometer measurement method or planar grating measurement method, the flexibility of the movable platform is neglected, and the movable platform is taken as a rigid body, and the six-degree-of-freedom displacement of the exposure region is calculated by measuring the displacement of other position on the movable platform, so that the error is larger when the rigidity of the movable platform is lower. In the prior art, it is possible to measure six degrees of freedom of the movable platform by an integrated reading head. However, the reading head of the existing planar grating measurement system is mostly placed on the movable platform, such that a displacement of a fixed point on the movable platform is measured, but the six-degree-of-freedom displacement of the constantly changed exposure region cannot be measured, during the movement of the wafer stage. In addition, there is a measuring method which measures the displacement of other position of the movable platform, has considered the flexible modality of the movable platform, and calculates in the exposure region in real-time. However, the method has lower precision and complicated algorithm.